1. Field of the Invention
The present invention relates to sheet conveyance devices for conveying sheets.
2. Description of the Related Art
Image forming apparatuses control conveyance of recording material and image formation processing so as to form an image at a predetermined position on the recording material. If the recording material is conveyed obliquely relative to a conveyance direction (skew) or conveyed in a state of being misaligned in a direction (lateral direction) perpendicular to the conveyance direction (lateral position error), the image is not formed at the predetermined position. U.S. Pat. No. 8,079,589 proposes a skew correction mechanism for correcting skew of recording material. Also, Japanese Patent Laid-Open No. 05-124752 proposes a correction mechanism for correcting lateral position error (lateral registration correction mechanism). Particularly, Japanese Patent Laid-Open No. 05-124752 proposes an invention that detects the position of a side edge of recording material using a line sensor provided on a conveyance path and shifts rollers in the lateral direction so that the side edge position is aligned with a predetermined position. Here, the “side edge” means a side of the recording material that is parallel to the conveyance direction, among four sides thereof.
To accurately align the side edge position with the predetermined positions, it is important to accurately detect the side edge position. Japanese Patent Laid-Open No. 2004-25579 describes an invention that detects the side edge position by detecting that the level of an output signal from a comparator changed from an output level of reflected light from the surface of recording material to an output level of reflected light from a black conveyance guide.
According to Japanese Patent Laid-Open No. 2004-25579, a position detection sensor is provided between a registration roller and a photosensitive drum on a conveyance path. An advantage of providing the position detection sensor at this position is that position detection can be executed for recording material after a flexure thereof formed due to skew correction is resolved and the recording material returns to being substantially flat. If the position detection is executed on recording material with a flexure, a detection error occurs because the distance from the position detection sensor to the surface of the recording material is not a same value.
Thus, the invention described in Japanese Patent Laid-Open No. 2004-25579 is advantageous regarding the accuracy of the side edge position detection, but the time taken from the detection to toner image transfer is significantly short because the side edge position is detected immediately before the photosensitive drum that transfers a toner image. Moreover, recording material is conveyed without stopping in a zone from the registration roller to the photosensitive drum, and therefore, this time needs to be set short still further. Meanwhile, side edge position detection processing includes adjustment of the light amount of a light source for irradiating recording material with light, determination of a threshold value for detecting the side edge position, detection of the side edge position, and determination of the shift amount of the registration roller for correcting the side edge position. Recording material conveyance speed is becoming higher in recent years, and it is therefore very difficult to execute the aforementioned processes in such a short time.
As a result, the inventor of the present application thought that a longer detection time can be ensured by arranging a sensor for detecting the side edge position between the registration roller and a conveyance roller that is located on the upstream side of the registration roller in the conveyance direction. In this case, the leading edge of the recording material abuts the registration roller and skew is corrected, and thereafter, the flexure is resolved by rotating the registration roller and returning the recording material to being flat, and the side edge position of the recording material that is being conveyed is detected. Here, if the length of the recording material in the conveyance direction is long, the time taken to pass through the detection position of the sensor becomes long, and the detection time can be thus ensured. However, in the case of recording material of the smallest conveyable size for the conveyance device, such as a postcard or a business card, the time taken to pass through the detection position of the sensor inevitably becomes short. Note that the smallest conveyable size depends on the distance between the registration roller and the conveyance roller that is located upstream of the registration roller. As mentioned above, in order to improve the accuracy of the side edge position detection, the detection can be executed after the flexure is resolved. However, in the case of recording material whose length in the conveyance direction is short, sufficient detection time cannot be ensured because the time taken to pass through the detection position after the flexure is resolved is too short.